U.S. patent number 10,191,352 [Application Number 15/451,852] was granted by the patent office on 2019-01-29 for optical component for modulating a light field and applications thereof.
This patent grant is currently assigned to Ludwig-Maximilians-Universitaet Muenchen, Max-Planck-Gesellschaft zur Foerderung der Wissenschaften e.V.. The grantee listed for this patent is Ludwig-Maximilians-Universitaet Muenchen, Max-Planck-Gesellschaft zur Foerderung der Wissenschaften e.V.. Invention is credited to Elena Fedulova, Kilian Fritsch, Vladimir Pervak, Oleg Pronin, Michael Trubetskov.
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United States Patent |
10,191,352 |
Fedulova , et al. |
January 29, 2019 |
Optical component for modulating a light field and applications
thereof
Abstract
Optical component (10) for modulating light field (1) incident
thereon, particularly amplitude and/or phase in dependency on
intensity (I) thereof, includes stack (11) of refractive layers
(12, 13) on substrate (14), made of materials having third-order
nonlinearity, and having alternatingly varying refractive indices
(n), including linear contribution (n.sub.0) and non-linear
contribution (n.sub.2), and determining reflectance and
transmittance spectra of the optical component, wherein refractive
layers (12, 13) are configured such that reflectance and
transmittance of the optical component have a Kerr effect based
dependency on intensity (I) of the incident light field with
n=n.sub.0+In.sub.2, and refractive layers (12, 13) are made of at
least one of dielectric and semiconductor layers, wherein
non-linear contribution (n.sub.2) is below 10.sup.-12 cm.sup.2/W. A
resonator device including the optical component, a method of
modulating a light field using the optical component and a method
of manufacturing the optical component are described.
Inventors: |
Fedulova; Elena (London,
GB), Pronin; Oleg (Garching, DE),
Trubetskov; Michael (Garching, DE), Fritsch;
Kilian (Eching, DE), Pervak; Vladimir (Munich,
DE) |
Applicant: |
Name |
City |
State |
Country |
Type |
Max-Planck-Gesellschaft zur Foerderung der Wissenschaften e.V.
Ludwig-Maximilians-Universitaet Muenchen |
Munich
Munich |
N/A
N/A |
DE
DE |
|
|
Assignee: |
Max-Planck-Gesellschaft zur
Foerderung der Wissenschaften e.V. (Munich, DE)
Ludwig-Maximilians-Universitaet Muenchen (Munich,
DE)
|
Family
ID: |
55527215 |
Appl.
No.: |
15/451,852 |
Filed: |
March 7, 2017 |
Prior Publication Data
|
|
|
|
Document
Identifier |
Publication Date |
|
US 20170261833 A1 |
Sep 14, 2017 |
|
Foreign Application Priority Data
|
|
|
|
|
Mar 9, 2016 [EP] |
|
|
16000571 |
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G02F
1/3511 (20130101); G02F 1/3556 (20130101); H01S
3/1112 (20130101); H01S 3/08059 (20130101); G02F
1/3523 (20130101); G02F 1/355 (20130101); G02F
1/21 (20130101); G02F 2203/15 (20130101) |
Current International
Class: |
G02F
1/01 (20060101); G02F 1/21 (20060101); G02F
1/35 (20060101); G02F 1/355 (20060101); H01S
3/11 (20060101); H01S 3/08 (20060101) |
Field of
Search: |
;359/258 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Other References
Nalwa, Hari Singh. "Organic Materials for Third-Order Nonlinear
Optics." Advanced Materials, vol. 5, No. 5, 1993, pp. 341-358.
cited by examiner .
Brzozowski et al. (2000). Optical signal processing using nonlinear
distributed feedback structures. IEEE Journal of Quantum
Electronics, 36(5): 550-555. cited by applicant .
Husakou et al. (2007). Steplike transmission of light through a
metal-dielectric multilayer structure due to an intensity-dependent
sign of the effective dielectric constant. Physical review letters,
99(12): 127402:1-127402:4. cited by applicant .
Grigoriev et al. (2010). Bistability, multistability and
non-reciprocal light propagation in Thue-Morse multilayered
structures. New Journal of Physics, 12(5), 053041: 1-20. cited by
applicant .
Leuthold et al. (2010). Nonlinear silicon photonics. Nature
Photonics, 4(8): 535-544. cited by applicant .
Lin et al. (2007). Nonlinear optical phenomena in silicon
waveguides: modeling and applications. Optics Express,
15(25):16604-16644. cited by applicant .
Moshonas et al. (2014). Simulation and properties of highly
nonlinear multilayer optical structures using the transmission line
method. SPIE Photonics Europe. International Society for Optics and
Photonics: 913129:1-913129:11. cited by applicant .
Tikhonravov et al. (1996). Application of the needle optimization
technique to the design of optical coatings. Applied optics,
35(28): 5493-5508. cited by applicant .
Tikhonravov et al. (2012). Modern design tools and a new paradigm
in optical coating design. Applied optics, 51(30): 7319-7332. cited
by applicant .
Tikhonravov, A. V. (2003). Design of optical coatings. In N. Kaiser
and H. K. Pulker (Eds.) "Optical interference coatings", Springer
81-104. cited by applicant .
Waldecker et al. (2015). Time-domain separation of optical
properties from structural transitions in resonantly bonded
materials. Nature materials, 14(10), 991-995. cited by applicant
.
English language abstract for DE 10109084 A1 (2002). cited by
applicant .
European Search Report from corresponding EP 16000571 dated Sep. 7,
2016. cited by applicant.
|
Primary Examiner: Alexander; William R
Assistant Examiner: Duong; Henry A
Attorney, Agent or Firm: Caesar Rivise, PC
Claims
What is claimed is:
1. An optical component, being configured for modulating a light
field incident on the optical component, wherein the light field
has a predetermined center wavelength, comprising a stack of
refractive layers being arranged on a substrate, being made of
materials having a third-order nonlinearity, and having
alternatingly varying refractive indices (n), including a linear
contribution (n.sub.0) and a non-linear contribution (n.sub.2), and
determining reflectance and transmittance spectra of the optical
component, wherein the refractive layers are configured such that
reflectance and transmittance of the optical component have a Kerr
effect based dependency on the intensity (I) of the incident light
field with n=n.sub.0+In.sub.2, wherein the refractive layers are
made of at least one of dielectric and semiconductor layers, and
the non-linear contribution (n.sub.2) is below 10.sup.-12
cm.sup.2/W, wherein the optical component has at least one of the
following features: the dielectric layers are made of at least one
of Ta.sub.2O.sub.5, Nb.sub.2O.sub.5, HfO.sub.2, TiO.sub.2,
ZrO.sub.2, Al.sub.2O.sub.3 and SiO.sub.2, and the semiconductor
layers are made of at least one of Ge, Si and ZnSe.
2. The optical component according to claim 1, wherein the
refractive layers are configured such that the reflectance and
transmittance spectra of the optical component have a slope section
including the center wavelength of the light field.
3. The optical component according to claim 2, wherein the slope
section has a slope steepness of at least 5.
4. The optical component according to claim 2, wherein the slope
section has a slope steepness of at least 7.
5. The optical component according to claim 2, wherein the
refractive layers are configured such that the slope section has
decreasing reflectance and increasing transmittance at increasing
wavelengths, and the reflectance and the transmittance of the
optical component are increasing and decreasing, respectively, at
increasing amplitudes of the incident light field.
6. The optical component according to claim 2, wherein the
refractive layers are configured such that the slope section has
increasing reflectance and decreasing transmittance at increasing
wavelengths, and the reflectance and the transmittance of the
optical component are decreasing and increasing, respectively, at
increasing amplitudes of the incident light field.
7. The optical component according to claim 2, wherein the slope
section of the reflectance and transmittance spectra is at least
one of an edge section and a resonance section of the reflectance
and transmittance spectra.
8. The optical component according to claim 1, wherein the
refractive layers are configured such that the reflectance and
transmittance spectra of the optical component have a plateau
section including the center wavelength of the light field.
9. The optical component according to claim 1, wherein the
refractive layers have at least one of the features a number of the
refractive layers is at least one of at least 2 and at most 1000,
and a thickness of the stack of refractive layers is at least 1 nm
and at most 1000 .mu.m.
10. The optical component according to claim 1, wherein the
refractive layers have at least one of the features a number of the
refractive layers is at least one of at least 10 and at most 100,
and a thickness of the stack of refractive layers is at least 5 nm
and at most 15 .mu.m.
11. The optical component according to claim 1, wherein the
substrate has a thickness below 500 .mu.m.
12. The optical component according to claim 1, wherein the
substrate has a thickness below 300 .mu.m.
13. The optical component according to claim 1, wherein the stack
of refractive layers includes at least one full-wave cavity being
resonant at least at the center wavelength of the light field.
14. The optical component according to claim 1, wherein the
refractive layers are configured such that the reflectance and
transmittance spectra include wavelengths in a range from 500 nm to
10 .mu.m.
15. An optical resonator device, including a plurality of cavity
mirrors spanning a resonator beam path, and at least one optical
component according to claim 1.
16. The optical resonator device according to claim 15, wherein the
at least one optical component is arranged as a reflective or as a
transmissive component.
17. A method of modulating a light field, comprising the steps of
directing the light field on an optical component according to
claim 1, and providing a modulated light field in at least one of
reflection and transmission relative to the optical component.
18. A method of manufacturing an optical component, being
configured for modulating a light field incident on the optical
component, wherein the light field has a predetermined center
wavelength, comprising depositing a stack of refractive layers on a
substrate, wherein the refractive layers are made of materials
having a third-order nonlinearity and having alternatingly varying
refractive indices (n), including a linear contribution (n.sub.0)
and a non-linear contribution (n.sub.2), and determining
reflectance and transmittance spectra of the optical component,
wherein the refractive layers are configured such reflectance and
transmittance of the optical component have a Kerr effect based
dependency on the intensity (I) of the incident light field with
n=n.sub.0+In.sub.2, the refractive layers are made of at least one
of dielectric and semiconductor layers, and the refractive layers
are configured such that the non-linear contribution (n.sub.2) is
below 10.sup.-12 cm.sup.2/W, and wherein the optical component has
at least one of the following features: the dielectric layers are
made of at least one of Ta.sub.2O.sub.5, Nb.sub.2O.sub.5,
HfO.sub.2, TiO.sub.2, ZrO.sub.2, Al.sub.2O.sub.3 and SiO.sub.2, and
the semiconductor layers are made of at least one of Ge, Si and
ZnSe.
19. The method according to claim 18, wherein the optical component
is configured for modulating at least one of an amplitude and a
phase of the light field in dependency on the intensity (I)
thereof.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
This application claims priority from EP 16 000 571.6 filed Mar. 9,
2016, which is incorporated herein by reference in its
entirety.
FIELD OF THE INVENTION
The present invention relates to an optical component for
modulating a light field, in particular for modulating at least one
of an amplitude and a phase of the light field, an optical
resonator device including the optical component, a method of
modulating a light field, in particular at least one of an
amplitude and a phase of the light field, e.g., in the optical
resonator device, and a method of manufacturing the optical
component. Applications of the invention are available e.g., in the
fields of ultrafast laser techniques.
BACKGROUND OF THE INVENTION
In the present specification, reference is made to the following
publications cited for illustrating prior art techniques, in
particular with regard to optically non-linear components and
applications thereof in ultrafast laser techniques. [1] L.
Brzozowski et al. in "IEEE J. Quantum Electron. 36(5), 550-555
(2000); [2] J. Leuthold et al. in "Nature Photon." 4, 535-544
(2010); [3] Q. Lin et al. in "Opt. Express" 15, 16604-16644 (2007);
[4] L. Waldecker et al. in "Nat. Mater. Nature Publishing Group"
(2015), NMAT4359; [5] V. Grigoriev et al. in "New Journal of
Physics" 12, 053041 1367-2630/10/053041 (2010); [6] N. Moshonas et
al. in "Proc. of SPIE" Vol. 9131 913129-1 (2014); [7] N. Kaiser and
H. K. Pulker "Optical interference coatings" Springer, 2013, A. V.
Tikhonravov, chapter "Design of Optical Coatings", pp. 81-104; [8]
A. V. Tikhonravov et al. in "Appl. Opt." 35, 5493-5508 (1996); and
[9] A. V. Tikhonravov et al. in "Appl. Opt." 51, 7319-7332
(2012).
It is generally known that optical multilayer coatings, e.g.,
allowing precise control of group delay dispersion (GDD)
characteristics over more than one octave or providing
high-damage-threshold broadband characteristics are incorporated
into the majority of ultrafast lasers, in particular femtosecond
lasers. Ultrafast lasers have enabled unprecedented quality in
micro-machining applications, have considerably increased medical
capabilities and have brought imaging techniques like multiphoton
fluorescence microscopy and optical coherence tomography to a new
level. Furthermore, ultrafast lasers are unique tools in research
applications in fields like attosecond physics, frequency comb
metrology and femtochemistry.
Current femtosecond laser oscillators yield average output powers
of several hundreds of W and pulse energies of several tens of
.mu.J at repetition rates of more than 10 MHz. These laser
parameters correspond to peak powers >50 MW, thus outperforming
even some laser-amplifiers. At such intense optical fields,
non-linearities in interference coatings may be revealed and
exploited. Various non-linear components are used in laser physics,
which can be divided into two general classes: firstly, components
based on non-linear absorption with finite relaxation time, and
secondly, components based on instantaneously (<5 fs time scale)
occurring non-linear response.
The first class includes components comprising a stack of
metallodielectric layers (MDS) and numerous kinds of (real)
saturable absorbers. MDS components have a significant non-linear
optical response so that they are suitable in particular for
providing non-linear mirrors having an amplitude dependent
reflectance. However, applications of these mirrors are restricted
to the visible spectral range, which is outside the range of
interest of multiple ultrafast laser applications. Furthermore, MDS
components may have disadvantages in terms of a step-like
dependency of the output intensity on the input and a slow recovery
after irradiation on a time scale of about 20 ps.
Semiconductor saturable absorber mirrors (SESAM) of the first class
have been widely used for mode-locking in laser oscillators,
however, finite relaxation time prevents the generation of pulses
shorter than 700 fs in Yb:YAG lasers. Moreover, they are strongly
wavelength dependent, difficult to grow and characterize, introduce
non-saturable losses, reveal two-photon absorption (TPA) processes,
have rather moderate modulation depth for high power operation
(less than 2%), have low damage threshold and are quite
expensive.
The second class includes artificial saturable absorber components
and components based on the instantaneous Kerr effect. Artificial
saturable absorber components include e.g., single-mode fibers
exploiting non-linear phase shifts for additive-pulse mode-locking,
or components using Kerr lensing in a gain medium. Due to broad
bandwidth, these techniques have advantages over SESAMs for
generating shorter pulses. Nevertheless, they impose serious
constraints on the cavity design and alignment sensitivity.
The nearly instantaneous nature of the electronic non-linear
response (due to Kerr effect and TPA) of silicon leads to many
applications related to high-speed optical-signal processing [2,
3]. As a further matter, a recent study [4] has shown that optical
modifications of a phase-change material Ge.sub.2Sb.sub.2Te.sub.5
(GST) are one order of magnitude larger than those achievable with
silicon and present a new route to high-speed optical modulators
for communications and computations. Furthermore, alternating
layers of materials possessing opposite Kerr non-linearities have
been modelled and analytically analyzed [5]. Dielectric
quasicrystals have been theoretically studied and found to
demonstrate an interplay between the intrinsic spatial asymmetry of
the structure and Kerr non-linearity causing bistability and
multistability of transmission (whereas the reflection has not been
studied) sensitive to the propagation direction [5]. Besides, the
presented components require an auxiliary pump signal in order to
have no limitations of the maximum value of transmission.
Another optical component of the second class, comprised of
successive thin-film layers possessing high values of non-linear
susceptibility, has been numerically investigated [6].
Specifically, optical bistability which translates to a change in
the value of reflectivity with the increasing input power might
have been observed. The authors of [6] considered doped polymer
films PPAA (push-pull aromatic azobenzene) possessing a non-linear
Kerr refractive index as high as about 1.7.times.10.sup.-6
cm.sup.2/W, which was plugged into the calculations as
-1.7.times.10.sup.-6 cm.sup.2/W and most probably have led to an
improper result. The behavior of another Bragg structure comprised
by 30 pairs of linear layers of transparent ceramic Sc.sub.2O.sub.3
and layers of antiferromagnetic Co.sub.3O.sub.4 having a non-linear
refractive index of 1.times.10.sup.-6 cm.sup.2/W at 405 nm has been
numerically studied in [6] as well. Moreover, the authors of [6]
predicted a hysteresis phenomenon when strong non-linearity is
present.
The structure described in [6] has not been put into practice,
probably due to simulating the non-linear response of exotic
materials, whose non-linear refractive indices are indeed rather
high, but unrealistic for practical applications. Moreover,
Moshonas et al. [6] have oversimplified the situation in the
calculations: The authors have not considered possible issues
related to nonlinear absorption processes, arising together with
the desired change of refractive index induced by the external
optical irradiation.
Objective of the Invention
The objectives of the invention are to provide an improved optical
component for modulating a light field, an improved optical
resonator device, an improved method of modulating a light field,
in particular in an optical resonator device, and/or an improved
method of manufacturing the optical component, which are capable of
avoiding disadvantages and limitations of conventional techniques.
In particular, the objective is to modulate a light field incident
on the optical component within an extended spectral range, in
particular in near infrared or infrared wavelength range, with a
time response on a reduced time scale, with improved capability of
precisely and reproducibly adjusting non-linear features and/or
with more prominent non-linear effects, while avoiding
laser-induced breakdown of the component.
SUMMARY OF THE INVENTION
The above objectives are solved by an optical component, an optical
resonator device and methods of the invention.
According to a first general aspect of the invention, the above
objective is solved by an optical component, which is configured
for modulating a light field incident on the optical component, in
particular for changing at least one of amplitude and phase of the
light field in reflection and/or transmission in dependency on the
amplitude of the incident field, and which comprises a stack of
refractive layers being arranged on a substrate. The refractive
layers are made of materials having a third-order non-linearity,
and they are arranged with alternatingly varying refractive
indices. Preferably, the refractive layers are arranged such that
the refractive indices are alternatingly increasing and decreasing.
The refractive index (n) of each refractive layer includes a linear
contribution (n.sub.0) and a non-linear contribution (n.sub.2). The
optical component, in particular the stack of refractive layers
thereof, has reflectance and transmittance spectra determined by
the materials, refractive indices and thicknesses of the refractive
layers.
Furthermore, the refractive layers are configured with taking into
account that reflectance (or: reflectivity) and transmittance (or:
transmissivity) of the optical component have a Kerr effect based
dependency on the intensity (I) of the incident light field with
n=n.sub.0+In.sub.2 (I being proportional to the square of the
amplitude).
According to the invention, the refractive layers are made of at
least one of dielectric and semiconductor layers.
Furthermore, the materials of the refractive layers are selected
such that the non-linear contribution (n.sub.2) of the refractive
indices is below 10.sup.-12 cm.sup.2/W.
According to a second general aspect of the invention, the above
objective is solved by an optical resonator device, including a
plurality of cavity mirrors spanning a resonator beam path, and at
least one optical component according to the above first general
aspect of the invention. Preferably, the at least one optical
component can be arranged as a reflective and/or as a transmissive
component in the optical resonator device. Advantageously, the
optical resonator device can be provided with a compact structure
while having new functions fulfilled by the at least one optical
component.
According to a third general aspect of the invention, the above
objective is solved by a method of modulating a light field,
preferably having a predetermined center wavelength in
near-infrared (NIR) or infrared (IR) spectral range, comprising
steps of directing the light field on the optical component
according to the above first general aspect of the invention and
providing the modulated light field in reflection and/or
transmission relative to the optical component. The incident light
field has an intensity such that the stack of the refractive layers
provides an optically non-linear response to the incident light.
The intensity of the incident light field is selected such that the
reflected or transmitted light has an intensity and/or phase, which
depends on the intensity of the incident light field. Preferably,
the intensity is just below a damage threshold of the stack of the
refractive layers.
According to a fourth general aspect of the invention, the above
objective is solved by a method of manufacturing the optical
component according to the above first general aspect of the
invention by depositing the stack of refractive layers on a
substrate. Advantageously, a broad range of deposition methods is
available which allow the production of dense layers with excellent
homogeneity, high reproducibility and high stability. Preferably,
magnetron beam sputtering is used for depositing the refractive
layers. Advantageously, this method allows a precise
time-controlled monitoring of deposition process parameters
resulting in an extremely high accuracy of layer thicknesses and
correspondingly an improved capability of precise and reproducible
non-linear modulation of light fields by the optical component.
Advantageously, the materials and non-linear refractive indices
selected for the inventive optical component result in a
well-defined increase or decrease of the component's reflective or
transmissive response and/or controlled change of light field
phase. The alternating layers of either dielectric or semiconductor
materials, or a combination thereof provide the inventive optical
component with a definite non-linear pre-damage behavior. The
inventors have found that with the non-linear refractive index
below 10.sup.-12 cm.sup.2/W, which is in contrast to higher
non-linear refractive indices of the structures considered in [6],
effects of non-linear absorption in the layer stack are suppressed.
Accordingly, temperature effects are significantly suppressed and
the optical component can be adapted for practical applications
with enhanced non-linear behavior, controlled GDD and increased
reliability and precision. Additionally, it has been demonstrated
that the non-linearity is based on the optical Kerr effect and
therefore has a very fast response time, preferably at least below
1 ps and particularly below 10 fs. With the currently available
characterization setup response time of about 1 ps has been
measured and is limited by the characterization laser pulse
duration. The inventive optical components prove useful for
mode-locking of solid state lasers, and can be used as spatial or
temporal (noise eater) filters, limiters and stabilizers.
Generally, a refractive index change (.DELTA.n) is always
associated with the non-degenerate non-linear absorption
(.DELTA..kappa.) and vice versa as stated by the Kramers-Kronig
transformation based on the principle of causality. Note that the
change of absorption coefficient (.DELTA..kappa.) may be negligibly
small. Thus, along with the desired change of refractive index, the
external optical perturbation inevitably causes the absorption
coefficient changes, which subsequently would lead to an
intensity-dependent temperature rise. Non-linear absorption
processes may even prevail for the response of the whole structure,
if materials with high values of n.sub.2 and correspondingly low
bandgap are used, e.g., as in [6]. On the contrary, for the
materials used with the inventive optical component, in particular
with values of n.sub.2 below 10.sup.-12 cm.sup.2/W (correspondingly
higher bandgap), multi-photon absorption can be avoided or
significantly suppressed. Naturally, this strongly depends on the
excitation wavelength (photon energy). The closer the photon energy
to the bandgap, the higher the probability of the multiphoton
absorption.
Advantageously, the inventive optical component is a multilayer
dielectric and/or semiconductor coating exhibiting a strong
non-linearity. This coating created on a substrate shows increasing
reflectance and decreasing transmissivity for increasing intensity
(or, in an alternative embodiment, correspondingly decreasing
reflectance and increasing transmittance for increasing intensity).
Accordingly, the inventive optical component is called Multilayer
Amplitude Modulator (MAM).
According to a preferred embodiment of the invention, the stack of
refractive layers is configured such that the reflectance and
transmittance spectra of the optical component have a slope section
including the center wavelength of the light field. Advantageously,
the observable non-linear change of reflectance and/or
transmittance is enhanced in the slope section.
A slope steepness m can be estimated by
m=|(B.sub.1-B.sub.2)/(A.sub.2-A.sub.1)|, where [A.sub.1, B.sub.1]
and [A.sub.2, B.sub.2] are the coordinates (values in arbitrary
units) of the highest and lowest parts of the spectrum curve
accordingly. Particularly preferred, the stack of refractive layers
is configured such the slope section has a slope steepness of at
least 5, in particular at least 7.
Advantageously, the refractive layers can be adapted such falling
or rising slopes of the reflectance/transmittance spectra and the
center wavelength of the incident light are matched relative to
each other. According to the first variant, the refractive layers
are configured such that the slope section has decreasing
reflectance and increasing transmittance at increasing wavelengths,
and the reflectance and the transmittance of the optical component
are increasing and decreasing, resp., at increasing amplitudes of
the incident light field. Alternatively, according to a second
variant, the refractive layers are configured such that the slope
section has increasing reflectance and decreasing transmittance at
increasing wavelengths, and the reflectance and the transmittance
of the optical component are decreasing and increasing, resp., at
increasing amplitudes of the incident light field. Advantageously,
different functions can be fulfilled by the optical component
adapted for one of the two variants. For instance, such a device
can be used for mode-locking and operate in transmission (laser
beam goes through the coating) or in reflection as one of the
folding resonator mirrors of an optical resonator.
According to further preferred features of the invention, the slope
section of the reflectance and transmittance spectra is at least
one of an edge section and a resonance section of the spectra. The
edge and resonance sections typically have different slopes.
Matching the center wavelength of the incident light and the at
least one of the edge and resonance sections allows an adaptation
of the non-linear effect to be obtained to the particular task of
the optical component.
Alternatively, the stack of refractive layers is configured such
that the center wavelength of the light field is included in the
middle of the reflectance and transmittance spectra of the optical
component (plateau section of the spectra). Advantageously, this
results in a broader bandwidth reflection or transmission
operation, compared with the embodiment where the center wavelength
of the light field is included in a slope section of the
spectra.
The stack of alternating refractive layers can be made of
dielectric materials, semiconductor materials, or dielectric
materials and semiconductor materials. Preferably, the dielectric
layers are made of at least one of Ta.sub.2O.sub.5,
Nb.sub.2O.sub.5, HfO.sub.2, TiO.sub.2, ZrO.sub.2, Al.sub.2O.sub.3
and SiO.sub.2, and/or the semiconductor layers are made of at least
one of Ge, Si and ZnSe. These materials have particular advantages
in terms of available deposition techniques, providing the
non-linear refractive index below 10.sup.-12 cm.sup.2/W and
available investigations of femtosecond pre-damage dynamics.
According to further preferred features of the invention, the
number of the refractive layers is at least 2, e.g., 5 or more, in
particular at least 10, and/or at most 1000, in particular at most
100. Furthermore, the thickness of the whole structure of the
refractive layers preferably is at least 1 nm, in particular at
least 5 nm, and/or at most 1000 .mu.m, in particular at most 15
.mu.m.
If, according to further particularly preferred feature of the
invention, the substrate has a thickness below 500 .mu.m, in
particular below 300 .mu.m, advantages in terms of minimizing
substrate effects on the function of the optical component and in
particular suppressing self-focusing in the substrate are obtained.
Preferably, the substrate is made of fused silica, sapphire, Si, or
ZnSe.
Advantageously, multiple design options are available for selecting
the thicknesses of the refractive layers. According to a first
variant, all refractive layers can have the same optical thickness.
According to further variants, all refractive layers of the same
material (dielectric or semiconductor) can have the same thickness,
or the refractive layers can have different thicknesses.
According to another, particularly preferred embodiment of the
invention, the stack of refractive layers can include at least one
full-wave cavity being resonant at least at the center wavelength
of the light field. In other words, the thickness of at least one
refractive layer and the center wavelength of the light field are
matched such that the respective at least one refractive layer and
the adjacent refractive layers with different refractive indices
provide a resonator inside the layer stack. Advantageously, the
resonator provides a field enhancement within the stack of
refractive layers, so that non-linear effects can be obtained even
with moderate intensities of incident light.
Advantageously, the invention allows overcoming the wavelength
limitations of conventional MDS components. Preferably, the
refractive layers of the inventive optical component are configured
such that the reflectance and transmittance spectra include a
wavelength in a range of at least 500 nm and/or at most 10 .mu.m.
With a preferred example, the refractive layers are adapted for
non-linear reflectance and/or transmittance for wavelengths of at
least 600 nm. Particularly preferred, the slope section of the
reflectance and/or transmittance is located in the range above 600
nm.
The inventors use the fact that the longer the wavelength, the less
probability of the multiphoton absorption and other
intensity-dependent absorption effects, which occur to be limiting
factors for non-linear optical processes. Non-linear response free
of intensity-dependent absorption, based purely on the
instantaneous change of the non-linear refractive index n.sub.2 is
obtained at longer wavelengths. Semiconductor materials are
especially interesting in the IR range due to their transparency
and high non-linear refractive indices. Besides, higher intensities
can be applied at longer wavelengths.
BRIEF DESCRIPTION OF THE DRAWINGS
Further details and advantages of the invention are described in
the following with reference to the attached drawings, which show
in:
FIGS. 1 to 3: schematic illustrations of stacks of refractive
layers according to preferred embodiments of the invention;
FIG. 4: graphical representations of changing reflectance spectra
with the inventive method of modulating an amplitude of a light
field;
FIG. 5: graphical representations of further features of the
optical component according to preferred embodiments of the
invention;
FIG. 6: a schematic illustration of a test set-up for
characterizing inventive optical components;
FIG. 7: graphical representations of further features of the
optical component according to preferred embodiments of the
invention; and
FIGS. 8 and 9: schematic illustrations of an optical resonator
device according to preferred embodiments of the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Features of preferred embodiments of the invention are described in
the following with reference to the design of the optical
components as illustrated in FIGS. 1 to 3 and the applications of
the optical components as illustrated in FIGS. 8 and 9. It is noted
that the implementation of the invention is not restricted to the
optical components illustrated in an exemplary manner. In
particular, embodiments of the invention can be modified with
regard to the number, thickness and material of the refractive
layers. Furthermore, the inventive optical components can be used
not only in optical resonators, but also in other optical set-ups,
e.g., laser amplifiers, or as laser spatial filters.
According to the invention, it is possible to exploit the optical
Kerr effect to provide components working in transmission and
components working in reflection. Exemplary reference is made to
optical components working in reflection. Optical components
working in transmission are configured in an analogue way.
The practical configuration of an inventive optical component (MAM)
is designed using a simulation software calculating the number of
the refractive layers, the refractive indices thereof and the
thicknesses thereof in dependency on the reflectance and/or
transmittance of the layer stack and the reflectance change to be
obtained. This calculation includes solving an optimization problem
which is based on introducing a merit function (F(X), see e.g.,
according to [7]), estimating the proximity of designed spectral
characteristics to target ones. The software solving the
optimization problem utilizes approaches such as needle
optimization and gradual evolution algorithms [8, 9], avoiding
problems of convergence at a local minimum. Subsequently, the
optical component is manufactured using the result of the
calculation. As an example, the simulation software comprises a
commercial OptiLayer software package (www.optilayer.com).
The inventive optical component has an intensity dependent
reflectance, transmittance and/or phase. The intensity of the
incident light field, which is required for obtaining this
optically non-linear behavior, is provided for each practical
implementation by numerical simulations on the basis of the
features of an optical component practically used, or by test
measurements with the optical component.
FIG. 1 schematically shows an embodiment of an optical component
10, comprising a stack 11 of refractive layers 12, 13 with
alternating refractive indices, being arranged on a substrate 14.
The refractive layers 12 (drawn with white lines, comprising e.g.,
semiconductors) have a higher refractive index compared with the
refractive layers 13 (drawn with black lines, comprising e.g.,
dielectrics). Alternatively, other materials can be combined, e.g.,
exclusively dielectric materials or exclusively semiconductor
materials, as well as dielectric/semiconductor structures.
The optical component 10 is arranged to be irradiated with incident
light 1 having a center wavelength and for reflecting (2) and/or
transmitting (3) the incident light. It is noted that the
reflected/transmitted light (2/3) is schematically illustrated
only. In practice, all of the refractive layers 12, 13 contribute
to the reflected light, i.e., the incident light partially
propagates through the stack 11. The reflected light 2 is a
superposition of all contribution from the refractive layers 12,
13.
With a practical example, the optical component 10 is produced
using magnetron sputtering technique (Helios, Leybold Optics). For
the optical component 10 working at a center wavelength 1030 nm,
the dielectric materials used for magnetron sputtering
manufacturing comprise Ta.sub.2O.sub.5, Nb.sub.2O.sub.5, and
SiO.sub.2 (e.g., 69 layers with a thickness of 9300 nm), while the
semiconductor materials comprise Ge and ZnSe (e.g., 70 layers with
a thickness of 8000 nm). Semiconductor materials such as Ge or Si
in combination with ZnSe can be used when creating optics for
longer wavelengths (around 2 .mu.m). The produced samples were
characterized with the help of an industrial spectrophotometer
(Perkin Elmer Lambda 950) and a home-built white-light
interferometer (WLI) (see FIG. 5, below).
FIGS. 2 and 3 schematically illustrate the optical thicknesses
(geometrical thickness multiplied with refractive index) within the
stack of refractive layers. According to FIG. 2, the refractive
layers have varying optical thicknesses, while FIG. 3 shows an
example, wherein a full-wave cavity 15 is formed within the stack
of refractive layers having identical optical thickness.
The optical component 10 is designed on the basis of the following
theoretical considerations. The optical Kerr effect is a non-linear
optical effect which occurs when intense light propagates in media
possessing third-order non-linearity. Using the Kerr effect has
particular advantages as it predicts an increase of reflectance for
higher intensities and ultrashort (several fs) time response--two
incredibly important parameters for modern ultrafast solid-state
lasers. The Kerr effect physically originates from a non-linear
polarization generated in the medium, which itself influences the
propagation properties of the light. It is the effect of an
instantaneously occurring non-linear response, which is described
by the following formula: n=n.sub.0+In.sub.2 (1) where n.sub.0 is
the linear refractive index, n.sub.2 is the second-order non-linear
refractive index of the medium, and I is intensity of the light.
Put simply, the refractive index growth consequently leads to
higher reflectance values. Therefore, non-linear effects are
observed before approaching the damage threshold of the
coating.
The non-linear refractive index depends on the proximity of a test
wavelength to the bandgap, and scales in proportion to the inverse
fourth power of the bandgap energy E.sub.g:
n.sub.2.about.E.sub.g.sup.-4 (2) For transparent crystals and
glasses, n.sub.2 is normally of the order of 10.sup.-16 cm.sup.2/W
to 10.sup.-14 cm.sup.2/W. Thorough studies of non-linear refractive
indices of the dielectric materials preferably used for the
inventive optical component have not been presented in the
available literature. The bandgap energies of high-index dielectric
materials Ta.sub.2O.sub.5, Nb.sub.2O.sub.5 and HfO.sub.2 are known
to be 3.8, 3.9 and 5.1 eV respectively, which allows one to
conclude that the typical values of n.sub.2 do lie between
10.sup.-16 cm.sup.2/W and 10.sup.-14 cm.sup.2/W.
Semiconductor materials possess rather high non-linear index
values. Si and Ge, for instance, are known to have strong
third-order non-linear optical coefficients with reduced TPA in the
IR spectral range. The non-linear refractive index of a
single-crystal ZnSe in the 1200-1950 nm region is known to be
between about 1.6.times.10.sup.-14 and about 0.9.times.10.sup.-14
cm.sup.2/W.
Depending on the relationship of the reflectance spectrum of the
optical component 10 and the center wavelength of the incident
light 1 (see FIG. 1), several types of optical components operating
in reflection can be provided, in particular optical components
working in the middle of the reflectance spectrum (type I), and
optical components working on a steep slope of the reflectance
spectrum (type II). The Kerr effect of types I and II is
illustrated in FIG. 4, which presents numerical simulations of
intensity dependent changes of reflectance spectra of a MAM, e.g.,
as shown in FIG. 1. The reflectance spectra are calculated with the
commercial OptiLayer software package (www.optilayer.com).
FIG. 4(a) shows an example of a broadband MAM working in the middle
(plateau section) of the reflectance zone (type I), while FIGS.
4(b) and 4(c) show examples of a MAM working at an edge structure
(long wavelength end of reflectance spectrum, type II) and a
resonant structure (within the reflectance spectrum, type II). The
type II design, in particular if the slope is at least 5, is
extremely sensitive to the change of refractive index. The GDD of
such structures is carefully tailored and remains nearly constant
in the desired wavelength range, e.g., if used for mode-locking of
lasers (see FIG. 5(b)). Additionally or alternatively, it is
possible to create an edge structure with an opposite slope
resulting in decreasing reflectance (increasing transmittance) for
increasing laser intensity. As an example, the MAM can work at an
edge structure at the short wavelength begin of reflectance
spectrum.
An important advantage of the type I MAM (FIG. 4(a)) designed for
reflection operation is its broad bandwidth, which can result in
the generation of pulses in the range of several fs, although an
absolute increase of reflectance can hardly exceed several percent.
The type II (FIGS. 4(b) and (c)) of MAM, on the other hand, is
extremely responsive to the intensity increase and can reach 20% in
particular cases (if the initial reflectance is 50%). Another
benefit of these sensitive designs is the possibility to work at
the edge itself, the position of which can be easily adjusted by
varying the angle of incidence and thus influencing the modulation
depth of the device. However, the estimated bandwidth of the
example "edge structure" design (FIG. 4(b)) is 10 nm, which would
limit the pulse durations down to 100-200 fs.
A basic example of a resonant structure (FIG. 4(c)) consists of two
stacks of refractive layers enclosing a full-wave cavity 15 in
between (Fabry-Perot filter, see FIG. 3). The advantage of the
resonant structure is the electric field enhancement obtained in
the cavity 15. The latter fact means that much lower intensities
are required for approaching non-linear regime.
It is noted, that another slope of a resonant structure can be used
to achieve the decreasing reflectance (increasing transmittance).
Furthermore, more complicated multiresonant structure embodiments,
including multiple cavities, can be designed in order to further
amplify the optical Kerr effect.
In analogy to FIG. 4, intensity dependent negative GDD values of
the optical component, in particular the stack of refractive layers
thereof, can be obtained. Accordingly, the phase of the incident
light can be controlled in dependency on the intensity of the
incident light.
As an example, the comparison of the designed and measured data for
the optical component of type II for reflection operation,
extremely responsive to the intensity increase and possessing
controlled negative GDD, is presented in FIG. 5. Corresponding
results can be obtained with the optical component of type I. FIG.
5(a) shows the designed transmittance (black dashed curve) and the
measured data (via spectrophotometer, grey circles) for 0.degree.
angle of incidence. FIG. 5(b) shows the designed GDD (black dashed
curve) and the measurement performed with a WLI (grey circles) for
20.degree. angle of incidence and p-polarized light. Control of GDD
characteristics additionally provides the control over the phase of
the light. Despite the challenging for fabrication design
(sensitive to manufacturing errors), the agreement between theory
and experiment is quite good. Some deviation can be noticed in FIG.
5, however, this is not important as it can easily be compensated
for by tuning the angle of incidence, adjusting the position of the
slope.
Increasing and controlled non-linear change of reflectance is one
of the main features employed with the inventive optical
components. A measurement setup 20 used for characterization of
sample components and for modulation depth measurements is shown in
FIG. 6. The measurement setup 20 includes a laser source 21 with an
Yb:YAG regenerative amplifier (p-polarization, central
wavelength=1030 nm, repetition rate=50 kHz and pulse duration=1
ps). The laser beam is focused on an optical component 10 in order
to provide intensities up to slightly below the damage threshold
thereof. The pulse intensity was varied with a half-wave plate
followed by a polarization cube placed in front of the focusing
lens. The reflected and transmitted average power values were
measured for each incident power value via a powermeter placed in
two positions 22, 23. The temperature of the sample was monitored
during the measurements as well (via IR camera 24). The sample was
placed on a rotational stage in order to adjust the angle of
incidence and achieve different initial
reflectance-to-transmittance ratios.
An example characterization of a sample component of type II (edge
structure), using the measurement setup 20 described is shown in
FIG. 7. The graphs reveal the evidence that the reflectance (R) of
the sample component increases by 7.+-.2%, the transmittance (T)
decreases by 9.3%.+-.0.4%, the relative temperature grows by
113.+-.2.degree. C., and the total losses are on a scale of the
experimental uncertainty for the case R.sub.0/T.sub.0=80%/20%
(right graph). In fact, the temperature effect is estimated to be
almost a half of the observed reflectance increase. Overall, the
experimentally observed pre-damage behavior of the studied optical
components occurs to be an interplay of the temperature-induced and
nonlinear effects in the multilayer structures. Nevertheless, it is
possible to almost fully circumvent the temperature influence by
introducing an optical modulator.
The increase of reflectance and decrease of transmittance depend on
the angle of incidence, which determines the initial proportion R
to T (R.sub.0/T.sub.0). As one can see (FIG. 7), the absolute
modifications of reflectance and transmittance are almost twice
higher for the case of R.sub.0/T.sub.0=50%/50% compared to those of
R.sub.0/T.sub.0=80%/20% for the same component of type II.
Additionally, with the measurement setup 20 described, it has been
found that the inventive optical components do not show hysteresis
phenomena with cycling increasing and decreasing intensities of
incident light. This represents a further advantage over the
structures described in [6].
Pump-probe measurements have shown the fast time response of the
inventive optical component. A conventional degenerative pump-probe
setup has been used with the laser source 21 of FIG. 6. Measuring
time-resolved recovery curves of the optical component of type II
(edge structure) for different initial R.sub.0/T.sub.0 at 85% of
damage threshold fluence provided evidence that the reflectance
change disappears very quickly after the pulse for all the initial
conditions. A response time of the optical component shorter than 1
ps was obtained.
Applications of the inventive optical component for oscillator
mode-locking are illustrated in FIGS. 8 and 9, which show optical
resonator devices 30 with a plurality of cavity mirrors 31, 32
spanning a resonator beam path (linear resonators). Alternatively,
ring resonators could be provided in analogue way. A mode-locked
oscillator can be operated in the regime of either negative or
positive dispersion. Dispersion compensation can be provided by a
set of dispersive mirrors 32. The inventive optical component 10
and a gain medium 33 (laser crystal) are included in the resonator
beam path. The laser gain medium can be chosen as a bulk crystal
medium, as a slab crystal and as a thin-disk crystal. In case of
thin-disk crystal medium this elements serves as one of the folding
mirrors (in reflection) previously known from state of the art. In
the optical resonator device 30, the optical component 10 is used
as a mode-locking device.
FIGS. 8 and 9 show typical mode-locked oscillator configurations
consisting of the laser gain medium 33, the linear or ring
resonator beam path, the highly reflective mirrors 31 (curved
mirrors of convex and concave shape) and dispersive mirrors 32.
According to FIG. 8 the optical component 10 is operated in
reflection, while a transmission configuration is shown in FIG.
9.
Preferably, the optical component 10 is positioned inside of the
resonator cavity where the beam size is small enough to provide
necessary intensity and thus cause the reflectance change. For
instance, the optical component 10 can be located in the beam focus
formed by two concave mirrors 31 as shown in FIG. 8 and
simultaneously serve as an output coupler.
Many other cavity configurations can be considered providing the
small beam size inside of the cavity. For instance, the small beam
size can be achieved closer to one of the end mirrors 32. In this
case, the optical component 10 can be used as an end-mirror or
positioned closer to the end mirror as shown in FIG. 9.
The optical component 10 can be used as a mode-locker in the
combination with other mode-locking techniques, for instance in
combination with KLM. Furthermore, more than one optical component
10 can be used inside of the laser cavity. Additional optical
components 10 can enhance the effect of self-amplitude modulation.
Moreover, complimentary MAMs can be inserted inside of the cavity
such that the overall bandwidth (or dispersion curve) provided by
two or more optical components 10 is better than that provided by
the single optical component 10.
The features of the invention disclosed in the above description,
the drawings and the claims can be of significance both
individually as well as in combination or sub-combination for the
realization of the invention in its various embodiments.
* * * * *